Lithography is the process of printing from specially prepared surfaces, some areas of which are capable of accepting lithographic ink, whereas other areas, when moistened with water, will not accept the ink. The areas which accept ink form the printing image areas and the ink-rejecting areas form the background areas.
In the art of photolithography, a photographic material is made imagewise receptive to oily or greasy inks in the photo-exposed (negative-working) or in the non-exposed areas (positive-working) on a hydrophilic background.
In the production of common lithographic printing plates, also called surface litho plates or planographic printing plates, a support that has affinity to water or obtains such affinity by chemical treatment is coated with a thin layer of a photosensitive composition. Coatings for that purpose include light-sensitive polymer layers containing diazo compounds, dichromate-sensitized hydrophilic colloids and a large variety of synthetic photopolymers. Particularly diazo-sensitized systems are widely used.
Upon imagewise exposure of the light-sensitive layer the exposed image areas become insoluble and the unexposed areas remain soluble. The plate is then developed with a suitable liquid to remove the diazonium salt or diazo resin in the unexposed areas.
Alternatively, printing plates are known that include a photosensitive coating that upon image-wise exposure is rendered soluble at the exposed areas. Subsequent development then removes the exposed areas. A typical example of such photosensitive coating is a quinone-diazide based coating.
Typically, the above described photographic materials from which the printing plates are made are camera-exposed through a photographic film that contains the image that is to be reproduced in a lithographic printing process. Such method of working is cumbersome and labor intensive. However, on the other hand, the printing plates thus obtained are of superior lithographic quality.
Attempts have thus been made to eliminate the need for a photographic film in the above process and in particular to obtain a printing plate directly from computer data representing the image to be reproduced. However the photosensitive coating is not sensitive enough to be directly exposed with a laser. Therefor it has been proposed to coat a silver halide layer on top of the photosensitive coating. The silver halide may then directly be exposed by means of a laser under the control of a computer. Subsequently, the silver halide layer is developed leaving a silver image on top of the photosensitive coating. That silver image then serves as a mask in an overall exposure of the photosensitive coating. After the overall exposure the silver image is removed and the photosensitive coating is developed. Such method is disclosed in for example JP-A-60-61 752 but has the disadvantage that a complex development and associated developing liquids are needed.
GB-1 492 070 discloses a method wherein a metal layer or a layer containing carbon black is provided on a photosensitive coating. This metal layer is then ablated by means of a laser so that an image mask on the photosensitive layer is obtained. The photosensitive layer is then overall exposed by UV-light through the image mask. After removal of the image mask, the photosensitive layer is developed to obtain a printing plate. This method however still has the disadvantage that the image mask has to be removed prior to development of the photosensitive layer by a cumbersome processing.
Furthermore methods are known for making printing plates involving the use of imaging elements that are heat-sensitive rather than photosensitive. A particular disadvantage of photosensitive imaging elements such as described above for making a printing plate is that they have to be shielded from the light. Furthermore they have a problem of sensitivity in view of the storage stability and they show a lower resolution. The trend towards heat mode printing plate precursors is clearly seen on the market.
For example, Research Disclosure no. 33303 of January 1992 discloses a heat mode imaging element comprising on a support a cross-linked hydrophilic layer containing thermoplastic polymer particles and an infrared absorbing pigment such as e.g. carbon black. By image-wise exposure to an infrared laser, the thermoplastic polymer particles are image-wise coagulated thereby rendering the surface of the imaging element at these areas ink-acceptant without any further development. A disadvantage of this method is that the printing plate obtained is easily damaged since the non-printing areas may become ink accepting when some pressure is applied thereto. Moreover, under critical conditions, the lithographic performance of such a printing plate may be poor and accordingly such printing plate has little lithographic printing latitude.
U.S. Pat. No. 4,708,925 discloses imaging elements including a photosensitive composition comprising an alkali-soluble novolac resin and an onium-salt. This composition may optionally contain an IR-sensitizer. After image-wise exposing said imaging element to UV- visible- or IR-radiation followed by a development step with an aqueous alkali liquid there is obtained a positive or negative working printing plate. The printing results of a lithographic plate obtained by irradiating and developing said imaging element are poor.
EP-A-625 728 discloses an imaging element comprising a layer which is sensitive to UV- and IR-irradiation and which may be positive or negative working. This layer comprises a resole resin, a novolac resin, a latent Bronsted acid and an IR-absorbing substance. The printing results of a lithographic plate obtained by irradiating and developing said imaging element are poor.
U.S. Pat. No. 5,340,699 is almost identical with EP-A-625 728 but discloses the method for obtaining a negative working IR-laser recording imaging element. The IR-sensitive layer comprises a resole resin, a novolac resin, a latent Bronsted acid and an IR-absorbing substance. The printing results of a lithographic plate obtained by irradiating and developing said imaging element are poor.
Furthermore EP-A-678 380 discloses a method wherein a protective layer is provided on a grained metal support underlying a laser-ablatable surface layer. Upon image-wise exposure the surface layer is fully ablated as well as some parts of the protective layer. The printing plate is then treated with a cleaning solution to remove the residu of the protective layer and thereby exposing the hydrophilic surface layer.
EP-A-573 092 discloses a method for making an image comprising the steps of:
image-wise exposing a heat mode recording material containing on a support (i) a recording layer containing a radiation to heat converting substance and an image forming substance and optionally (ii) a surface layer thereby PA1 decomposing said recording layer and optionally said surface layer in the exposed areas and PA1 rubbing said heat mode recording material to remove said optional surface layer and said recording layer in the exposed areas. PA1 (a) a photo-thermal conversion material, and PA1 (b) a high molecular compound, of which the solubility in an alkali developer is changeable mainly by a change other than a chemical change.
Said material is not developed with a developing solution.
EP-A-823 327 discloses a positive photosensitive composition showing a difference in solubility in alkali developer as between an exposed portion and a non-exposed portion, which comprises as components inducing the difference in solubility,
FR-A-1 561 957 discloses a processus in order of registering or reproducing information by means of electromagnetic radiation and also discloses elements sensible for heat containing substances wherein heat is produced by exposure to electromagnetic radiation.
U.S. Pat. No. 5,641,608 discloses a process for the direct production of an imaged pattern of resist on a substrate, which process utilizes thermo-resist rather than photoresist, i.e. compositions which undergo thermally-induced, rather than photo-induced, chemical transformations. A film of thermo-resist composition applied to the surface substrate is scanned by a focused heat source in a predetermined pattern, without a phototool, to bring about localized thermally-induced chemical transformations of the composition which either directly produce the resist pattern or produce in the film a developable latent image of the pattern.
GB-A-1 208 415 discloses a method of recording information comprising information-wise heating a recording material comprising a support bearing, with or without an inter-layer, a heat-sensitive recording layer constituted so that such information-wise heating creates a record of the information in terms of a difference in the water-permeability of different areas of the recording layer, treating the recording material with an aqueous liquid which penetrates through the water-permeable or more water permeable areas of the recrding layer and is constituted so as to effect a permanent physical and/or chemical change of at least the surface portions of the underlying support or inter-layer in the corresponding areas, and removing the whole of the recording layer to expose said information-wise changed underlying support or inter-layer.
U.S. Pat. No. 5,466,557 discloses a radiation sensitive composition especially adapted to prepare a lithographic printing plate that is sensitive to both ultraviolet and infrared radiation and capable of functioning in either a positive-working or negative-working manner, comprised of (1) a resole resin, (2) a novolac resin, (3) a latent Bronsted acid, (4) an infrared absorber, and (5) terephthalaldehyde. The solubility of the composition in aqueous alkaline developing solution is both reduced in exposed areas and increased in unexposed areas by the steps of imagewise exposure to activating radiation and heating.
GB-A-1 245 924 discloses an information-recording method wherein a recording material is used comprising a heat-sensitive recording layer of a composition such that the solubility of any given area of the layer in a given solvent can be increased by heating that area of the layer, wherein the said layer is information-wise heated to produce a record of the information in terms of a difference in the solubilities in the said solvent of different areas of the recording layer, and wherein the whole layer is then contacted with such solvent to cause the portions of the recording layer which are soluble or most soluble in such solvent to be removed or penetrated by such solvent, the said method being characterized in that the said recording layer is wholly or mainly composed of one or more heat-sensitive polymeric compounds.
GB-A-1 155 035 discloses a method of recording information, wherein a recording material is used comprising a layer of a polymeric material which when any given area of the layer is sufficiently heated undergoes in that area a modificarion resulting in a decrease in the solubility of that area of the layer in water or an aqueous medium, such layer also incorporating a substance or substances distributed over the whole area of the layer and being capable of being heated by exposing the layer to intense radiant energy which is absorbed by such substance or substances, and wherein the said material is exposed to intense radiant energy which is distributed over the material in a pattern determined by the information to be recorded and which is at least partly absorbed by said distributed substance or substances, so that a corresponding heat pattern is generated in the material, whereby such information is recorded in terms of a difference in the solubilities in water or an aqueous medium of different areas of said layer.
EP-A-97 200 588.8 discloses a heat mode imaging element for making lithographic printing plates comprising on a lithographic base having a hydrophilic surface an intermediate layer comprising a polymer, soluble in an aqueous alkaline solution and a top layer that is sensitive to IR-radiation wherein said top layer upon exposure to IR-radiation has a decreased or increased capacity for being penetrated and/or solubilised by an aqueous alkaline solution.
Said last heat-mode imaging element has the disadvantage that the sensitivity is marginal. An analogous material with a higher IR-sensitivity would be appreciated.